A man who allegedly killed his grandmother and critically injured his grandfather in a violent stabbing in south-east New South Wales yesterday has been charged with murder.

Police charged the 20-year-old man in Bega, on the state's South Coast, following a crime spree in which he allegedly attacked both of his grandparents and stole their car.

The man then later allegedly robbed a third man of his vehicle by hitting him in the head with a hammer.

After he was arrested at around 8.20pm, the alleged attacker was taken to hospital under police guard for assessment. Picture: 9NEWS. (9NEWS)Forensics officers remain on the scene investigating the alleged incident. Picture: 9NEWS. (9NEWS)

He has since been charged with one count of murder and two counts of wound with intent to murder.

NSW Police confirmed in a statement today that a 71-year-old man flagged down a passing car on East Street in Bega around 3.35pm after he had been stabbed at his home.

Emergency services who attended the scene discovered both he and his 69-year-old wife were critically injured.

The man's wife later died at the South East Regional Hospital, while he was airlifted to Canberra Hospital in a critical condition.

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Authorities from the South Coast Police District then began a pursuit of a car belonging to the couple, that been taken by their 20-year-old grandson suspected of the attack.

The man's grandmother died in hospital after the alleged attack, his grandfather and the third victim remain in a critical condition in hospital. Picture: 9NEWS (NIne)The 20-year-old is accused of hitting the third man in the head with a hammer, before also stealing his car and crashing it and then fleeing. Picture: 9NEWS. (Nine)

Local resident Rebecca Lupton told 9NEWS she had witnessed the 71-year-old man waving at cars, but thought it may have been him saying hello to someone rather than looking for help.

"We came home yesterday afternoon after the school pick-up, and as we drove around the corner I saw the older guy waving," she said.

"In hindsight that was when he was flagging down help because minutes later there was ambulances and police sirens.

"We're used to hoons coming down the street, we're not quite used to everything halting when the sound of that police siren starts... It's really scary, we're not used to that at all, we're used to being more casual."

At 4.40pm, it is believed the same 20-year-old allegedly struck another 55-year-old man in the head with a hammer while attempting to steal his car on Sapphire Coast Drive in Bournda.

Emergency services who attended the scene discovered both he and his 69-year-old wife were critically injured.
(Nine)

That man has also been airlifted to St George Hospital in a critical condition, while a female passenger in the car escaped without injury.

The 20-year-old thought to be behind the carjacking crashed the vehicle 500m away from the scene of the robbery and then ran into bushland.

A large-scale search was mounted by specialist and local police officers and the man was later arrested around 8.20pm and taken to hospital for assessment.

Inspector Sue Charman-Horton today told media that the incident is "unusual" for the community but praised the strength of Bega residents.

"This is a very unusual event for places like Bega, specifically here," she said.

Authorities from the South Coast Police District then began a pursuit of a car belonging to the couple, that been taken by their 20-year-old grandson suspected of the attack.
(Nine)

"Our staff put in so much effort and we know that the community will be supporting those families who are involved in this because of the strength of the community that we have."

During the search, residents in the area were told to lock all windows and doors, secure all keys to motor vehicles and to “be vigilant and avoid the area”.

Authorities are now investigating the circumstances of the alleged crime spree and have urged anyone in the public with information to assist them.